Do checkbox inputs only post data if they're checked?

Question

Is it standard behaviour for browsers to only send the checkbox input value data if it is checked upon form submission?

And if no value data is supplied, is the defa

Answer 1

Is it standard behaviour for browsers to only send the checkbox input value data if it is checked upon form submission?

Yes, because otherwise there'd be no solid way of determining if the checkbox was actually checked or not (if it changed the value, the case may exist when your desired value if it were checked would be the same as the one that it was swapped to).

And if no value data is supplied, is the default value always "on"?

Other answers confirm that "on" is the default. However, if you are not interested in the value, just use:

if (isset($_POST['the_checkbox'])){
    // name="the_checkbox" is checked
}